Mechanics L6 PPQs

Question 1

An experiment involves measuring the time taken for a ball to fall through different distances. The maximum distance is 1.5 m. One student says that it is better to use light gates or sensors and a data logger. Another student says that just using a stopwatch better. Discuss the advantages and disadvantages of each method. [4]

Answer 1

• light gates:
• Power needed – disadvantage
• No need to account for reaction time
• Reading to the nearest millisecond

Stopwatch:
- Simple to operate/set up
- Graph would have to be drawn manually
- Cheaper, easily possible

Question 2

The student is asked to determine a value for the acceleration of freefall (g) by timing, a falling steel ball. (Core practical with electromagnet and switch)
Explain what date will be collected and how it will be used to determine g [5]

Answer 2

• Times for at least five Heights
• Graph should be a straight line
• Determine gradient
• Use s=1/2 at^2
• Multiply gradient by two

Question 3

The student is asked to determine a value for the acceleration of freefall (g) by timing, a falling steel ball. (Core practical with electromagnet and switch)
Identify the main sources of uncertainty and/or systematic error [2]

Answer 3

• time taken by electromagnet to release ball
• Parallax in specified measurement
• 0 error on height or timer

Question 4

The student is asked to determine a value for the acceleration of freefall (g) by timing, a falling steel ball. (Core practical with electromagnet and switch)
Comment on safety [1]

Answer 4

• dont tread on ball
• wear shoes
• no major hazards bc low voltage

Question 5

To determine g, the student uses the equation v^2=u^2+2as
Explain why a graph of v^2 on the Y axis and s on the x axis should be a straight line through the origin [3]

Answer 5

• a = constant, a=g
• v^2=2as —> y=mx
• v^2 is directionally proportional to s provided a=constant and u=0

Question 6

State one assumption that must be made when calculating the work done against air resistance [1]

Answer 6

• frictional forces are negligible
  -force of gravity + air resistance are the only significant forces acting to oppose the motion

Question 7

In a bungee jump, the jumper falls from a high platform while attached to an elastic cord. The cord is also attached to the platform. The cord slows the bungee jumper down, so that he comes to rest before reaching the ground.
The fall can be divided into three stages:
1 - the jumper is in free fall until the cord starts to stretch
2 - the cord is stretching until the acceleration of the jumper decreases to zero
3 - the cord continues to stretch until the jumper is momentarily at rest
Explain, in terms of work done, how the kinetic energy of the bungee jumper changes during the three stages of the fall [6]

Answer 7

1: KE increases of the jumper
Because work is done by gravitational force
2: KE of jumper is increasing but at a decreasing rate
Bc work is done on the cord (as it stretches)
3: KE of jumper is decreasing
Bc work is done on the cord at a greater rate than the gravitational force does work on the jumper
OR he comes to rest bc the total work done by gravitational forces is equal to work done stretching the bungee.

Question 8

Equivalent to 1 kilowatt-hour
A 0.28 J
B 0.28 W
C 3.6 x 10^6 J
D 3.6 x 10^6 W

Answer 8

C
1x10^3 x 60 x 60

Question 9

What quantity has the SI base unit kgm2s-3

Answer 9

Power

Question 10

A water pump causes 200g of water to be ejected from the nozzle of a garden house each second at a velocity of 3m/s

Find an expression that could be used to determine the minimum output power inwatts required from the pump

Answer 10

0.2 x 3^2 /2

P=wd/t
KE = wd
1/2mv^2

Question 11

‘If you hold an apple in your hand it’s about a newton, if you raise it through one metre, that;s about a joule and if you do it in one second, that’s about a watt’

Assuming that the apple has a mass of 100g, explain and justify the statements made about newton. Joule and watt
[6]

Answer 11

• w=mg
• w = o.1 x 9.81 = 1N
• GPE=mgh , 0.1 x 9.81 x 1 = 0.98J
• P=wd/t
• P= 0.98/1 = 0.98W

Question 12

A student is investigating projectiles. He fires two small identical balls, A and B, simultaneously.
(A is thrown from a height above the x axis with a horizontal velocity. B is thrown from the floor with both vertical and horizontal components.)
The balls land at the same instant at the target , T

The paths AT and BT have different lengths. However, balls A and B take the same time to reach target T. Explain how this is possible. [4/5]

Answer 12

• Average velocity of A is greater than B.
• Bc A continually accelerates whereas B slows down
• Va = horizontal component of Vb because they travel the same horizontal distance
• vertical component of projectile motion does not affect the horizontal component

Question 13

When asked to run one complete lap around a track, a student says, ‘however fast I run, my average velocity for the lap will be zero’
Comment on his statement [3]

Answer 13

• correct because his displacement is zero because he finishes where he started
• velocity is a vector quantity
• velocity = displacement/ time

Question 14

A car is driven around a bend at a constant speed.
Explain what happens to its velocity [2]

Answer 14

• direction is changing
• so velocity is changing

Question 15

A coin is flicked off a table so that it initially leaves the table travelling in a horizontal direction with a speed of 1.5 m/s.

A coin of greater mass is flicked with the same horizontal speed. Compare the paths of the coins. Explain your answer
[3/4]

Answer 15

• follows same path
• all objects have the same acceleration due to gravity
• horizontal motion is unaffected by any force ( it is independent of vertical component)

Question 16

Martha throws two stones with the same force, one stone has a mass of 30g the other 60g. Explain which stone will have the greater initial acceleration.
[2]

Answer 16

60g will have a lower initial acceleration
Because a = inversely proportional to mass for a fixed resultant force

Question 17

A student is travelling forwards at a constant speed on a skateboard. The student jumps forward off the skateboard
Explain how newtons laws of motion apply to the student and the skateboard as he jumps forward off the moving skateboard [6]

Answer 17

• student applies a force to the skateboard
• by N3L, the skateboard also applies a force to the student
• the two forces are equal in size and opposite in direction
• according to N2L, the student accelerates
• the skateboard decelerates according to N2L
• the skateboard has a higher magnitude of acceleration than the student as it has a smaller mass.

Question 18

Two small identical solid metal spheres, A and B, are suspended by light inextensible threads from a frame.
Sphere A is pulled to one side and released. Sphere A collides with sphere B and stops and sphere B swings upwards.
Using newtons laws of motion, explain the motion of the spheres during the collision in terms of the forces acting on them. [6]

Answer 18

• sphere A applies a force to sphere B
• by N3L, sphere B applies a force to sphere A
• the two forces are equal in size and opposite in direction
• according to N2L, sphere A decelerates
• the force opposes the motion of sphere A, therefore it decelerates
• according to N2L, sphere B accelerates
• the magnitude of acceleration is equal because the spheres are identical meaning they have the same mass.

Question 19

When stout is poured, it contains many spherical bubbles of gas which rise and form the foamy ‘head’ at the top of the drink. The manufacturers of the drink state ‘it takes 120 seconds for the head to form’.
A bubble, initially at rest in the liquid, accelerated until it achieves a uniform upward velocity.
Explain the motion of the bubble [4]

Answer 19

• initially drag is zero + upthrust is greater than weight
• unbalanced forces so accelerates upwards
• as velocity increases, drag increases
• until drag + weight = upthrust
• so RF = 0, a=0 , v= constant

Question 20

A trebuchet is a medieval catapult designed to project a rock over large distances. The rock is held in a sling. When the peg is removed the counter weight falls and the rock is projected through the air. Student assumes that all the gravitational potential energy transferred from the counter weight is transferred to the kinetic energy of the rock.
Give three reasons why this is not correct.

Answer 20

• work is done against friction and energy is transferred to the surroundings.
• some kinetic energy is going into the beam/sling/counterweight.
• beam/ sling gain GPE

Question 21

A trebuchet is a medieval catapult designed to project a rock over large distances. The rock is held in a sling. When the peg is removed the counter weight falls and the rock is projected through the air

The mass of the counter weight was reduced. The trebuchet was then used to project the rock again. Explain why the horizontal distance travelled by the rock decreases.

Answer 21

• counterweight transfers less GPE
• so transfer of Ek is reduced
• therefore rock has smaller vertical component of velocity
• time of flight will be reduced
• horizontal component of velocity is smaller.

Question 22

A ball is dropped and bounces
Using the law of conservation of energy, explain the pattern of the ball’s path.

Answer 22

• high GPE
• ball falls, accelerates under gravity, increasing magnitude of velocity and Ek.
• on floor, GPE = 0
• bounces: velocity decelerates, Ek decreases, GPE increases
• energy is being transferred between GPE and Ek
• max height decreases with each bounce
• energy cannot be created or destroyed
• energy is transferred as thermal energy.

Question 23

(Velocity time graph to show a car that accelerates and then decelerates (shape of graph is like a mountain))
Discuss, with reference to the graph, the factors that would be used to predict the motion of the car over the 120s
[6]

Answer 23

• at higher speeds the force of air resistance is greater.
• this will cause the resultant force and therefore the acceleration to decrease.
• ignition of the rocket engine will cause the resultant force to increase.
• as fuel is used up, mass decreases and therefore acceleration increases.
• when brakes are applied, resultant force will be backwards and vehicle will decelerate.

Question 24

Explain why the assumptions about the acceleration in a SUVAT equation may not be correct in practice [2]

Answer 24

• air resistance increases with speed so acceleration decreases (at higher speeds)
• car could brake with greater negative acceleration than positive acceleration

Question 25

A box is dropped from a plane flying at a constant velocity and height
Assuming that air resistance is negligible, as the box falls to the ground, its horizontal position will … [1]

Answer 25

Remain directly under the plane

Question 26

A trebuchet is a medieval catapult designed to project a rock over large distances. The rock is held in a sling. When the peg is removed the counterweight falls and the rock is projected through the air.
The mass of the counterweight was reduced. The trebuchet was then used to project the rock again. Explain why the horizontal distance travelled by the rock decreases [5]

Answer 26

• counterweight transfers less GPE
• transfer of KE to rock is reduced
• rock has smaller vertical velocity
• time of flight reduced
• horizontal component of velocity will be smaller

Question 27

A trebuchet is a medieval catapult designed to project a rock over large distances. The rock is held in a sling. When the peg is removed the counterweight falls and the rock is projected through the air.
A student assumes that all the GPE transferred from the counterweight is transferred to the KE of the rock
Give two reasons why this is not correct [2]

Answer 27

• some energy will be transferred to the KE of the counterweight
• work is done against friction, transferring energy into the surroundings
• beam/rock also gains GPE

Question 28

The vertical acceleration of a gymnast varies while she is in contact with the trampoline.
Explain how the forces on the gymnast affect the vertical acceleration while she is in contact with the trampoline.
Your answer should identify the forces acting on the gymnast and the directions of the forces. Ignore air resitance. [6]

Answer 28

• the weight of the gymnast acts downwards on the gymnast.
• the normal contact force from the trampoline acts upwards.
• the normal contact force increases as she moves downwards.
• the normal contact force is maximum at the bottom of the bounce.
• resultant force is the difference between weight and normal contact force.
• when the normal contact force is greater than the weight, the acceleration is upwards.

Question 29

State the principle of conservation of momentum [2]

Answer 29

• total momentum before = total momentum after
• provided no external force acts on the system

Question 30

The KE of two skaters after the collision is much less than their combined KE before the collision, so the collision is inelastic.
Explain the decrease in KE in this collision

Answer 30

• forces acred between skaters during the collision
• work done by forces transfers KE to thermal energy

Question 31

A student investigated how the supporting forces on a bridge vary as traffic moves across the bridge. The student made a simple model of a bridge using a metre rule. The metre rule rested on two supports P and Q.
The upward force on the metre rule at P was Fp. The upward force on the metre rule at Q was Fq. A load was placed on the metre rule a distance x from support P. Forces Fp and Fq were measured for different values of x. Explain how Fp and Fq changed as x was increased.

Answer 31

For equilibrium, the clockwise and anticlockwise moment about P must equal. As x increases, the clockwise moment of the load about P increases. Fq must increase to increase the anti-clockwise moment. For equilibrium, the resultant force must be zero. Therefore as Fq increases, Fp must decrease

Question 32

Cricket player wears padded protective equipment. This protective equipment reduces the risk of injury if they are struck by the cricket ball. Explain how this protective equipment reduces the risk of injury to the player. Use ideas about momentum in your answer. [3]

Answer 32

• there is a change of momentum
• increases time
• decreases rate of change of momentum
• decreases force
• decreases pressure

Question 33

A body initially at rest, explodes into two masses M1 and M2. These masses move apart with speeds v1 and v2 respectively. The ration v1/v2 is equal to [1]

Answer 33

M2/M1

Question 34

Two identical spheres of mass m are both travelling with a speed v towards each other. The spheres collide head-on. Which of the following statements MUST be true after the collision?
A - total momentum = 2mv
B - total momentum = 0
C - total kinetic energy = mv^2
D - total kinetic energy = 0

Answer 34

B

Question 35

A building has five floors. The windows on successive floors are separated by the same vertical distance. A brick is dropped from a window on each floor at the same time. The bricks should hit the ground at
A decreasing time intervals
B equal time intervals
C increasing time intervals
D the same time

Answer 35

A

Question 36

Duirng a lesson on newtons laws of motion, a student says, ‘we dont really need to bother with newtons first law because it is included in his second law.’
State Newton’s first two laws of motion and explain how newtons second law includes the first
[5]

Answer 36

• N1L: constant velocity
• unless unbalanced
• N2L: EF= ma
  = EF = 0 , therefore a = 0

Question 37

In a demonstration of energy transfer, a large pendulum is made by suspending a 7.0kg bowling ball on a long piece of wire.
A student is invited to pull the ball back until it just touches her nose and then to release it and stand perfectly still while waiting for the ball to return.
The following instructions are given:
Do not push the ball - just release it.
Do not move your face before the ball returns.

Explain this demonstration and the need for these instructions [6]

Answer 37

• it will not strike the student’s face
• the total energy of the pendulum is constant (energy is conserved)
• it cannot move higher than its starting point because that would require extra GPE
• energy transfer GPE->KE
• energy dissipated against air resistance
• will stop it quite reaching its starting point
• pushing provides extra energy
• if pushed, it can move higher
• will hit student
• if face moves forward the ball may reach it before it is at its maximum height.

Question 38

The Charpy test is used by scientists to measure the fracture toughness of a material.
A simple pendulum, with a hammer on the end, is held high and released so that it swings down and strikes the sample. The height from which the hammer is released is increased until the sample fractures. Some energy is absorbed by the sample in the impact but the hammer continues to move until it comes to rest at the top of its swing. Due to the law of conservation of energy the hammer will not swing up as high as its starting position.

The difference in height between the start and end is proportional to the energy absorbed in the impact - the fracture toughness.
Justify this statement [2]

Answer 38

• energy absorbed in the impact is equal to the change in GPE (between the start and end)
• △E = mg△h And mg is constant so E is proportional to △h

Question 39

A motor is used to lift an object. The object is raised through a vertical height of 75cm at a constant speed of 0.4m/s. Which of the following gives the rate of increase of potential energy in watts?
A 0.25 x 9.81 x 0.40
B 0.25 x 0.75
C 0.25 x 9.81 x 0.75
D 0.5 x 0.25 x (0.4)^2

Answer 39

A

P = mgh/t = mgv

Question 40

When a male skater pushes a female skater forwards, the total kinetic energy of the skaters increases. Explain why kinetic energy is not conserved in this interaction. [2]

Answer 40

• male skater does work as he pushes female skater
• so there is an increase in KE

Question 41

Grasshoppers with longer legs accelerate to their launch velocity over a longer time.
Leg length has a negligible effect on both the mass of a grasshopper and the energy released in a jump.
Explain how leg length affects the force exerted on the ground during a jump. [4]

Answer 41

• energy released does not change, so same final velocity
• same increase in velocity over longer time means a smaller acceleration
• smaller force exerted by legs during jump
• so smaller force on ground

Question 42

In a recent study it was discovered that grasshoppers, living in an environment with hunting spiders, increase their launch velocity on average by 20%. The jump length of these grasshoppers was more than doubled.
Assess whether a 20% increase in launch velocity alone is sufficient to double the jump length [4]

Answer 42

Vertical velocity component increases by a factor of 1.2 so time in air increases by a factor of 1.2.
V = d/t
D = vt
1.2 x 1.2 = 1.44
Jump length increases by 44%